The present invention relates to calibration techniques for determining the characteristics of a laser beam, particularly for use with laser eye surgery systems. More specifically, the invention provides devices, systems, and methods for determining the dimensions and/or position of the laser beam spot upon a target, and can provide input for generating, verifying, or adjusting ablation algorithms used to plan a resculpting procedure. When used in conjunction with laser eye surgery systems, the present invention can assist in determining patterns of laser beam spot delivery upon a patient""s cornea, and can also be used in calibrating the laser beam delivery system.
When performing laser eye surgery such as when ablating a target region on a patient""s cornea with a refractive laser beam system, it is beneficial to have accurate information on the dimensions of the laser beam spot which is incident on the cornea. Deviation from a desired spot size and shape, such as by increased or decreased diameter of the laser beam spot or by the spot exhibiting an oval or non-symmetrical shape, could result in tissue ablation at undesired locations on the patient""s corneas with each laser pulse, leading to less than ideal resculpting. Inaccuracy in the location of the laser spots may result in off-center ablations.
The present invention provides methods and apparati for determining characteristics of a laser beam spot, the characteristics typically including the intensity, dimensions, and/or position of the laser beam spot. An advantage of the present invention is that it can be used with laser eye surgery systems such that the dimensions of the laser beam spot, (including its diameter, area and eccentricity), can be precisely determined prior to, or concurrently with, the laser beam spot being used to ablate a region of the patient""s cornea.
In preferred methods of the present invention, a laser beam is scanned in a path across a reference-edge, (which may preferably comprise a knife-edge), having a photodetector positioned therebehind, with the laser beam preferably remaining in a path generally perpendicular to the plane of the reference-edge during the scanning.
An output signal is generated by the photodetector corresponding to a percentage of the laser beam which is actually incident on the photodetector, (ie: not blocked by the reference-edge), at various moments in time during the scanning of the laser beam. For a beam having a uniform energy distribution, the percentage of the laser beam energy which is incident on the photodetector will correspond to the area of the laser beam spot which is incident on the photodetector. By measuring the output signal characteristics of the photodetector during the scanning, the present invention provides systems for determining the size and shape of the laser beam spot as well as the intensity of the laser beam. In preferred aspects, a computer calculates the intensity and shape profiles of the laser beam from the photodetector output signals.
As stated, the output signal generated by the photodetector will correspond to the size of the area of the laser beam spot incident thereon. As such, when the laser beam is fully incident on the reference-edge, (ie: when it is blocked from reaching the photodetector by the reference-edge), the photodetector will generate no output signal, or it will only generate a minimal output signal as a result of noise. Conversely, when the laser beam spot has been scanned completely across the reference-edge and is then fully incident on the photodetector, the photodetector will generate a maximum output signal.
The larger the area of the laser beam spot incident upon the photodetector, the stronger the output signal generated by the photodetector. Accordingly, in a preferred aspect of the invention, the intensity of the laser beam is determined by measuring the maximum output signal of the photodetector when the laser beam spot is fully incident on the photodetector and is not blocked by the reference-edge.
In another preferred aspect of the invention, the total area of the laser beam spot is determined by integrating the area under a curve representing the intensity of the photodetector signal output during the scanning as the laser beam is scanned across the reference-edge.
In yet another preferred aspect of the invention, the position of the center of the laser beam spot is located by determining when the output signal of the photodetector reaches half of its maximum output signal during the scanning, thus indicating that the center of the laser beam spot is positioned directly at the edge of the reference-edge, (with one half of the laser beam spot incident on the photodetector and one The larger the area of the laser beam spot incident upon the photodetector, the stronger the output signal generated by the photodetector. Accordingly, in a preferred aspect of the invention, the intensity of the laser beam is determined by measuring the maximum output signal of the photodetector when the laser beam spot is fully incident on the photodetector and is not blocked by the reference-edge.
In another preferred aspect of the invention, the total area of the laser beam spot is determined by integrating the area under a curve representing the intensity of the photodetector signal output during the scanning as the laser beam is scanned across the reference-edge.
In yet another preferred aspect of the invention, the position of the center of the laser beam spot is located by determining when the output signal of the photodetector reaches half of its maximum output signal during the scanning, thus indicating that the center of the laser beam spot is positioned directly at the edge of the reference-edge, (with one half of the laser beam spot incident on the photodetector and one half of the laser beam spot incident on the reference-edge).
In another preferred aspect of the present invention, the width of the laser beam spot in the direction of the path of the scanning is determined by locating the positions of the leading and trailing edges of the laser beam spot and then determining a spacing therebetween. In this aspect of the invention, the leading edge of the laser beam spot is located by determining when the photodetector begins to emit an output signal, (being indicative of the laser beam spot leading edge first passing over the reference-edge and becoming incident on the photodetector). The trailing edge of the laser beam spot is located by determining when the output signal of the photodetector has reached a maximum (indicating that the laser beam spot is not blocked by the reference-edge and is therefore fully incident on the photodetector). After determining the moments in time when the leading and trailing edges of the laser beam spot pass over the reference-edge as set out above, the width of the laser beam spot in the direction of the scanning is calculated based upon the speed of the laser beam scanning across the reference-edge.
In another preferred aspect of the present invention, the width of the laser beam spot in the direction of the path of the scanning is determined by locating the positions of the leading and trailing edges of the laser beam spot and then determining a spacing therebetween. In this aspect of the invention, the leading edge of the laser beam spot is located by determining when the photodetector begins to emit an output signal, (being indicative of the laser beam spot leading edge first passing over the reference-edge and becoming incident on the photodetector). The trailing edge of the laser beam spot is located by determining when the output signal of the photodetector has reached a maximum (indicating that the laser beam spot is not blocked by the reference-edge and is therefore fully incident on the photodetector). After determining the moments in time when the leading and trailing edges of the laser beam spot pass over the reference-edge as set out above, the width of the laser beam spot in the direction of the scanning is calculated based upon the speed of the laser beam scanning across the reference-edge.
In other aspects of the present invention, asymmetries and eccentricities in the laser beam spot are found by measuring the rate of change or the symmetry of the rate of change of the output signal during the scanning.
In yet other aspects of the present invention, the size, shape and position of the laser beam spot are determined in two directions which are preferably perpendicular to one another. In this aspect of the invention, scanning is preferably performed in two perpendicular paths, over perpendicular first and second reference-edges. In this aspect of the invention, the size, shape and position of the laser beam spot are determined in the two perpendicular directions by measuring the output signals from either a single photodetector or two separate photodetectors positioned behind the reference-edges. An advantage of this aspect of the invention is that asymmetries of the beam spot (ie: an irregular shape of the beam spot) as well as eccentricities of the beam spot (ie: elongation of the beam spot to form an oval-shape), can be detected.
In preferred aspects of the present invention, the photodetector is a bulk detector. As such, an advantage of the present invention is that a more complex and expensive imaging detector is not required.
The present invention also provides methods of calibrating scanning laser beam delivery system. These methods comprise positioning a calibration tool at a target location; directing the laser beam onto the tool; sensing the laser beam using the tool; and adjusting the system in response to the sensed laser beam. In various aspects, the laser beam can be repeatedly re-directed, (for example, by a galvanometric mirror), between the tool and a patient""s cornea. As such, after determining the size, shape and/or position of the beam, the laser beam can be applied at a known location on the cornea. Alternatively, the tool can be repeatedly inserted into and removed from the beam path between the laser beam source and the patient""s cornea. As such, the alignment tool can then be repeatedly removed from the target location to allow for resculpting of the patient""s cornea and then replaced at the target location after the resculpting of the cornea. Using either approach, a repetitive measurement of intensity and shape characteristics of the laser beam can be made as well as repetitive recallibration of the targeting of the laser beam can be achieved, thus ensuring precise positional accuracy when ablating the patient""s cornea.
In still further aspects of the invention, the laser beam is split with a first portion of the beam directed to the measurement/alignment tool and a second portion directed to the patient""s cornea such that real time measurement of shape and intensity characteristics of the laser beam spot and/or real time alignment of the laser beam delivery system can be achieved.
Regardless of the tool positioning, the calibration tool will often provide signals indicating beam spot size, shape, energy distribution, and/or location. These signals may be used to adjust the planned ablation protocol of the beam delivery system. Specifically, using the sensed information, an algorithm for calculating the locations and number of shots can be revised, thereby increasing the accuracy of the resculpting procedure. This calibration information can be used to adjust the ablation algorithm immediately before and/or during each ablation procedure.
In other aspects of the present invention, the measuring/alignment tool comprises a target which fluoresces in response to laser light incident thereon. In this second embodiment of the invention, an operator views the position of the fluoresced spot on the target screen while directing laser light at the target screen. Such viewing may preferably be done through the system microscope. The beam delivery system is aligned with the targeting optics, which may comprise a cross-hair reticle, thereby calibrating the laser beam delivery system.